1. Field of the Invention
The present invention relates to a compact zoom lens system for still cameras of a new lens-moving type including a field from the wide angle to the long focal length.
2. Description of the Prior Art
Several types of zoom lenses including a field from the wide angle to the long focal length have been heretofore known as follows:
(1) old type or so-called mechanically-corrected type zoom lens,
(2) zoom lens of the type composed of four movable lens groups having power of positive, negative, positive and positive, in which a first lens group and a fourth lens group in the (1) zoom lens as described above are moved,
(3) zoom lens composed of three movable lens groups having power of positive, negative and positive,
(4) wide-angle type zoom lens in which a lens group having a negative power precedes, and
(5) zoom lens composed of five movable lens groups having power of positive, negative, positive, negative and positive.
In the case a zoom lens system is desired to be obtained which has a focal length shorter than a diagonal line of an image plane at the wide position and has a focal length longer than a two-fold of a diagonal line of an image plane at the tele position, as in the present invention, these zoom lenses of the types as described above have the following problems though they have merits and demerits.
In the zoom lens of the (1) type, a diameter of the front lens becomes large to increase the lens system as a whole.
In the zoom lens of the (2) type, the lens system is rendered compact readily as compared with that of the (1) type but the structure of the lens system is unsymmetrical as positive, negative, positive and positive, which makes it difficult to correct distortion aberration and coma aberration.
In the zoom lens of the (3) type, since the structure is simple, the lens system must be enlarged or power of the individual lens groups must be strengthened in order to increase the zooming ratio, and if the power is strengthened for compactness, correction of aberrations becomes difficult.
In the zoom lens of the (4) type, this basically has the structure of the retrofocus type which is difficult to correct spherical aberration at the tele position.
In the zoom lens of the (5) type, the structure of lens system is symmetrical and many lens groups contribute to magnification variation and thus this zoom lens is suitable to obtain a large zooming ratio while keeping compactness. However, since five lens groups are independently moved, the construction of the lens frame unavoidably becomes complicated.